1. Field of the Invention
The present invention relates to an electrical switch. In particular, the present invention relates to a single-pole solid state electrical switch which can be directly connected to the AC power lines.
2. Discussion of the Related Art
The basic electrical circuit includes a power switch, and a load connected in series with the power switch, connected across the output terminals of a power source (e.g., an AC outlet). Typically, the power switch is a mechanical device which makes or breaks an electrical contact. The electrical contact is made or broken by a mechanical force provided either manually or through a magnetic field (e.g., a relay). A manually operated conventional mechanical switch typically toggles between xe2x80x9cONxe2x80x9d (conducting) and xe2x80x9cOFFxe2x80x9d (non-conducting) states by a mechanism of levers and springs. Such a power switch has typically a low reliability and a short life-time, especially when operating in a hostile environment (e.g., inflammable explosive, high temperature, high humidity, dusty or corrosive atmosphere, and severe vibrations). Such a power switch is susceptible to failures resulting from electrical arcing, sparks, mechanical wearout, corrosion, wetting, contact welding or contact miss. Under some circumstances, a failure of a power switch can lead to fires and other industrial accidents, endangering property and lives. To improve performance of these conventional mechanical switches, expensive modifications and use of precious metals are often required. Such improved switches remain susceptible to wearing out and frequent maintenance. Load damage can result from a defective mechanical switch.
Typically, in a conventional power switch, if the power switch is in the xe2x80x9cONxe2x80x9d state when the interruption occurs, the power switch does not reset itself to the xe2x80x9cOFFxe2x80x9d state after a power interruption. Under such circumstances, accidents can often occur when the supply of power is resumed unexpectedly. For safety reasons, in many heavy machinery, magnetic contactors are provided to reset the power switch. Such magnetic contactors typically are cumbersome, expensive and complicated, dissipate power and create xe2x80x9clow frequency humxe2x80x9d noise. In areas where the power supply is noisy, i.e., where there are frequent transient xe2x80x9csnap-offsxe2x80x9d and xe2x80x9cpower slotsxe2x80x9d, the corresponding frequent resets required for conventional mechanical switches create significant inconvenience.
In a conventional mechanical circuit breaker, the electromagnetic tripping or/and thermal tripping mechanism for overcurrent protection is not designed for frequent operations. When used with conventional mechanical switches in an electrical circuit with many branches, such a circuit breaker does not individually provide overcurrent protection to branch circuits and terminal loads. Typically, in most home or office applications, a master circuit breaker provides overcurrent protection to a large number of switches, so that overcurrent in one circuit results in shutting off a large number of circuits protected by the same overcurrent protection.
For over-voltage protection, a solid state device of the prior art (e.g., a solid state switch) is typically protected by a protective device which can be either a varister or a special thyristor connected in parallel with the solid state switch. In the xe2x80x9coffxe2x80x9d state of the solid state switch, when a substantial over-voltage occurs (e.g., when the voltage across the solid state switch exceeds the xe2x80x9cbreakoverxe2x80x9d voltage of the thyristor), the protective device becomes conducting to limit the voltage drop across the solid state device, thus protecting the solid state switch from damage by the over-voltage. However, if the over-voltage persists, the high current in the protective device can generate sufficient heat to irreversibly destroy the protective device over time. Thus, such an over-voltage protection scheme is expensive both because of the cost of the protective device and also for the cost of replacing the protective device.
In addition, conventional switches are not practical for implementing multi-point xe2x80x9crandomxe2x80x9d control (i.e., to allow switching xe2x80x9cONxe2x80x9d or xe2x80x9cOFFxe2x80x9d of a piece of machinery at any one of multiple locations) beyond three control points, because of the complex switch logic and the large number of wires that are required.
Because of the cost and the above disadvantages of the conventional switch, a solid state electrical switch is long desired. However, until now, one fundamental technical problem has not been solvedxe2x80x94a solid state electrical switch is necessarily an electronic circuit. As an electronic circuit, a DC power supply is typically required. In most integrated circuits, such a DC power supply operates at one or more lower DC supply voltages, such as 2.7 v, 3.3 v, 5 v, or xc2x15 v, xc2x112 v, . . . , xc2x135 v, . . . between power pins VCC (or VDD) and ground (or VSS). Thus, unless a battery provides the supply voltages, a power supply circuit is necessary to provide the operating voltages. As a power switch, which is typically connected in series with a load, such a power supply circuit necessarily draws a current through the load, in the form of a leakage current. Such a leakage current, even though from several milliamperes to tens of milliamperes, in fact, operates the load under an xe2x80x9cundervoltage condition.xe2x80x9d While such a switch may still be acceptable, for example, as a electronic dimmer in a lighting application, such a switch would be unacceptable, especially from a safety point of view, in applications such as fluorescent lights, AC motors, transformers or other appliances. For example, under the safety standards in virtually all countries and recognized safety organizations (e.g., the Underwriter Laboratories), a power switch which allows a leakage current in the milliampere range or higher is considered unsafe. In fact, for this reason, dimmers and electronic timers, even though connected serially with the lighting, are considered electronic appliances or loads rather than power switches. In many applications, where safety is a paramount concern, an additional conventional mechanical switch is often required to be provided in series with the electronic dimmer or timer.
In the prior art, without exception, the electronic circuit of a 2-terminal solid state switch is connected in parallel to the load current-conducting component (e.g., between the two anodes of a triode AC switch, or TRIAC). Examples of these switches can be found, for example, in U.S. Pat. Nos. 5,550,463 and 5,030,890. Thus, these switches draw a significant current during the solid state switch""s xe2x80x9coffxe2x80x9d state. From a switch current requirement point of view, any one of such 2-terminal solid state switches is not different from a solid state switch which draws a current through a third terminal directly coupled to the power source.
In addition, in the xe2x80x9cONxe2x80x9d state of a solid state switch of the prior art, the voltage drop across the solid state switch (e.g., across a TRIAC), Von-sat, is typically 0.8 to 1.8 volts AC. Thus, the electronic circuit of the solid state switch, which is connected in parallel with the switch terminals (i.e., across Von-sat), does not receive sufficient rail-to-rail voltage for proper operation. Alternatively, for example in U.S. Pat. No. 3,660,688 and 4,289,980, to obtain the operating voltages from the two terminals of the solid state switch, the voltage drops Von-sat""s across the solid state switch can be maintained at the higher voltage ranges of 2.4-4.0 volts and 12-14 volts, respectively. However, in those solid state switches, the power dissipation can be significant. For example, if one of the solid state switches of U.S. Pat. Nos. 3,660,688 and 4,289,980 is used in series with a 120 volts, 5-amp light fixture, the power dissipation in the solid state switch would reach 12-20 watts, in one case, and 60-70 watts, in the other case! To handle such severe power dissipation, not only are bulky heat sinks required, the resulting low performance and insufficient operating voltages across the load render such solid state switches impractical and undesirable.
In FIG. 1 of U.S. Pat. No. 4,703194 to Brovelli (xe2x80x9cBrovellixe2x80x9d), a 2-terminal network is disclosed. However, as in the prior art solid state switches discussed above, the main switch formed by the rectifier bridge (i.e., diodes 1-4) and the silicon controlled rectifier (SCR) 6 provide an xe2x80x9cONxe2x80x9d state voltage drop Von-sat of 2.4-4.0 volts. Thus, as in the solid state switches discussed above, a load current of 6 amperes would result in a power dissipation of 13-24 watts across the solid state switch. Further, to avoid SCR 6 from switching off when the AC voltage crosses zero volts, the xe2x80x9cONxe2x80x9d state of Brovelli""s solid state switch is maintained by the charge stored in capacitor 7. Capacitor 7 maintains a voltage (e.g., 0.7 volts) exceeding the trigger voltage of SCR 6. However, polarized capacitor 5, which Brovelli requires a 1-xcexcf electrolytic capacitor and performs a low-pass filtering function for the load current, cannot be used to sustain a load current exceeding one ampere. Under normal xe2x80x9cOFFxe2x80x9d-state operation, an electrolytic capacitor working on high voltage and high ripple current conditions, or a harsh surge power line, the leakage current flowing into the load can cause a breakdown, leading to undesirable and unpredictable results.
Further, to ensure that the solid state switch has high sensitivity, SCR 6 must be of high sensitivity also. Typically, because of the high sensitivity required, the gain of SCR 6 is relatively low, and thus can carry only a relatively small current (e.g., TIC106D SCR is rated for a current of about 1 ampere). In order to provide a higher current, a high power component, such as a TRIAC, must be included in the solid state switch. However, such a TRIAC would short the anode and cathode terminals of SCR 6, draining charge from capacitor 7 at the gate terminal of SCR 6, so that the xe2x80x9cONxe2x80x9d state of SCR 6 cannot be maintained when the input voltage crosses zero. Thus, Brovelli""s design cannot be used with practical currents, and cannot be extended to handle a larger current by simply including a high power component.
Furthermore, Brovelli""s solid state switch is turned on and off by triggering highly sensitive SCRs 6 and 9 through small currents created in touch plates 15 or 14 through resistors 8 and 11 respectively. Currently commercially available high-sensitivity SCRs (e.g., Mitsubishi""s CR02AM and CR03AM, and Motorola""s MCR100-8 and TIC106D) all require at least 200 xcexcA to trigger. However, when a human body contacts a touch plate, such as touch plate 14, the impedance between the touch plate and ground through the human body can often be as high as 100 megaohm, thereby providing a current much lower than 200 xcexcA and insufficient to trigger SCR6 or SCR9 to effectuate turning Brovelli""s solid state switch xe2x80x9cONxe2x80x9d or xe2x80x9cOFFxe2x80x9d. At other times, the resulting impedance between the ground and the touch plate through the human body can cause a current exceeding 200 xcexcA, thereby causing electric shocks or raising other safety issues. For these reasons, Brovelli""s solid state switch is deemed impractical.
The present invention provides a fully solid state 2-terminal electrical switch (referred to as the xe2x80x9cLiu Switchxe2x80x9d), which can be used with a single pole application (i.e., the load and the switch are coupled in series to an AC power line). The Liu Switch is a static switch which does not include any mechanical or moving component, and therefore is not susceptible to wear and tear. As the Liu Switch does not include mechanical contact points, it does not create a spark, an arc, corrosion, or mechanical noise, and can withstand operations in a hostile environment, such as a high temperature, high humidity, corrosive, dusty or intensely vibrating environment.
In one embodiment of the present invention, the Liu Switch, which can be directly connected in series with a load and an AC power outlet, includes (a) a semiconductor switch device controlled at a control terminal by a control signal which determines whether the semiconductor switch is in a conducting or non-conducting mode; (b) a rectifier receiving an AC signal from the terminals of the semiconductor switch device during the non-conducting mode of the semiconductor switch; and (c) a control circuit including a capacitor which (i) is coupled to receive the rectified signal of the rectifier during the non-conducting mode of the semiconductor switch and (ii) is discharged in response to an electrical signal from a gain circuit coupled in parallel to said capacitor.
In one implementation, during the non-conducting mode of the semiconductor switch, the rectified DC signal maintains the capacitor in a fully charged condition. The semiconductor switch remains in the non-conducting mode until the electrical signal which causes the capacitor to discharge is received. The electrical signal which causes the capacitor to discharge can be, for example, an electrical signal associated with a button being pressed. After the capacitor is discharged, the rectified DC signal provides a charging current to bring the capacitor back to a fully charged state. This charging current then provides the control signal, in the form of a trigger signal, to put the semiconductor switch into a conducting mode. The conducting semiconductor switch causes the capacitor to discharge. However, at each zero-crossing of the AC signal, the semiconductor switch device momentarily becomes non-conducting again, so as to allow the rectified DC signal to charge the capacitor. The charging current then regenerates the trigger signal to put the semiconductor switch device back into the conducting mode. Thus, once the semiconductor switch device is in a conducting mode, a regenerative or feedback process provides a control signal (e.g., the trigger signal) that ensures that the semiconductor switch device remains in the conducting mode.
In one implementation, the control circuit further includes a second gain circuit responsive to a second electrical signal. The second electrical signal causes a signal path to be provided between the control terminal and a common ground of the control circuit, thus interrupting the feedback process by shunting the control or trigger signal to ground.
The control circuit further includes an initialization circuit having a capacitor (the xe2x80x9csecond capacitorxe2x80x9d) coupled between the control terminal and the common ground. The second capacitor has a capacitance larger than the capacitance of the capacitor of the control circuit (the xe2x80x9cfirst capacitorxe2x80x9d). A forward-biased diode couples the control terminal to the second capacitor. A resistor connected in parallel with the second capacitor, in combination with the rest of the control circuit forms a circuit (xe2x80x9cLiu""s Networkxe2x80x9d) with multiple time constants. In one the embodiment, Liu""s Network serves as both a state memory and an initialization circuit. At initialization (e.g., when power is first applied), the second capacitor of Liu""s Network provides a large capacitance which absorbs the initial charging current of the first capacitor. Thus, the trigger signal that places the semiconductor switch device into the conducting mode is prevented. As a result, the Liu Switch remains in a non-conducting mode upon initialization.
Further, upon a power interruption occurring when the Liu Switch is a conducting mode, the Liu Switch remains in the conducting mode if the power resumes after a time period less than a predetermined time interval, and becomes non-conducting when the power interruption lasts longer than the predetermined time interval. Within the predetermined time interval, Liu""s Network serves as a state memory which retains the conducting or non-conducting mode of the Liu Switch prior to the power interruption. In one implementation, the second capacitor is realized by an electrolytic capacitor and an unpolarized capacitor coupled in parallel.
In one implementation of the Liu Switch, the control circuit further includes a second gain circuit having a terminal which receives an external signal. Absent the external signal (e.g., a trigger signal generated by a button being pushed), the second gain circuit does not draw any current and has a high output impedance.
According to one aspect of the present invention, the Liu Switch further includes a zero-crossing detection circuit coupled to receive the rectified signal and coupled to the control terminal. The zero-crossing detection circuit prevents the control signal from being asserted except when the instantaneous magnitude of the rectified signal is below a predetermined voltage. In one implementation, the zero-crossing detection circuit includes a transistor which shorts the control terminal to common ground when the instantaneous magnitude of the rectified signal rises above the predetermined value. In one implementation, the zero-crossing detection circuit is implemented by a transistor controlled by an output signal of a voltage divider between an output terminal of the rectifier and a common ground.
In addition, a light-emitting diode (LED) and a Zener diode connected in series with the voltage divider can be included. The LED can serve as a xe2x80x9cnight lightxe2x80x9d to allow the electrical switch to be visible for certain applications.
According to another aspect of the present invention, the Liu Switch includes a current detector coupled in series with the load and the semiconductor switch to provide a signal indicative of the current in the current detector. In one embodiment, the Liu Switch further includes an overcurrent protection circuit which forces the semiconductor switch into a non-conducting mode when the current detector indicates a current exceeding a predetermined value. The current detector can be implemented by a transformer. In one embodiment, the overcurrent protection circuit includes temperature-sensitive components, so that the threshold for overcurrent protection circuit can be self-tracking and adapted in accordance with the temperature of the environment and the temperature of the Liu Switch itself.
In one embodiment, the overcurrent protection circuit includes (a) a rectifier receiving a signal indicative of the current in the current detector to provide a voltage signal which represents the current in the current detector; and (b) a threshold component which becomes conducting when the magnitude of the current in the current detector exceeds a predetermined value. The threshold component can be implemented by a silicon diode, a Zener diode or a four-layer Shockley diode. The rectifier of the overcurrent protection circuit can be implemented by a Zener diode, or a diode bridge. Further, the overcurrent protection circuit can include a resistor network between the rectifier and the threshold component. This resistor network can include temperature-sensitive devices (e.g., thermisters or other thermal devices) which compensate and further fine-tune the overcurrent protection circuit""s temperature response. By appropriately selecting the temperature characteristics of the temperature-sensitive devices, the tripping condition of the overcurrent protection circuit can be automatically adjusted according to the temperature of the operating environment and the temperature of the switch.
In accordance with the present invention, the Liu Switch further includes an optocoupler which controls a Liu Switch in response to any one of multiple external signals received at various points of a control bus, thus providing xe2x80x9cmulti-point random controlxe2x80x9d to the Liu Switch.
In one implementation, the Liu Switch is provided by a diode bridge and a silicon controlled rectifier (SCR). In a second implementation, the semiconductor switch is implemented by a TRIAC. In a third implementation, the semiconductor switch is implemented by antiparallel silicon controlled rectifiers. The rectifier circuit of the Liu Switch can be provided by a SCR controlled bridge rectifier. A low-pass filtering circuit can be coupled to a signal terminal of the semiconductor switch device to further protect the semiconductor switch, by absorbing any surge, shock or noise in the control input signal, thus keeping the system in steadily operations.
The touch panels can each include a metallic surface, or a metallic surface coated with a resistive material or an insulator. The touch panel can be mounted in a plane offset from a mounting plate (e.g., in a shallow depression or provided slightly protruding over the surface of the mounting plate). In one implementation, where two touch panels (one for the xe2x80x9cONxe2x80x9d function and one for the xe2x80x9cOFFxe2x80x9d function) are provided, the touch panels are provided different colors or provided different tactile feels.
According to another aspect of the present invention, the Liu Switch includes a beep circuit for providing an audible response, in the form of a xe2x80x9cbeepxe2x80x9d sound, to the external agent when the agent contacts a touch panel. The beep sound response can be provided by a Zener diode and a piezoelectric speaker connected in series across an output terminal of the rectifier of the Liu Switch and a common ground. The beep circuit can generate audible and distinguishable beep sounds to indicate which of the two touch panels is contacted.
In the control circuit of the Liu Switch, the various components (e.g., the gain circuits, the SCR controlled rectifier, the semiconductor switches, or the audio response circuit) are each selected such that, during the xe2x80x9coffxe2x80x9d state of the semiconductor switch, the leakage current in each component is negligible. Consequently, negligible power is drawn by the Liu Switch during its xe2x80x9coffxe2x80x9d state.
Another advantage of the present invention is a state-latched control contact panels which retain the xe2x80x9cONxe2x80x9d or xe2x80x9cOFFxe2x80x9d state after contact by the external agent is broken. Based on this latch function, the Liu Switch of the present invention provides a multipoint random remote control system, including: (a) a 2-terminal Liu Switch coupled in series with a load circuit between two lines of an AC power outlet; (b) an optocoupler coupled to the Liu Switch, the optocoupler providing a very high electrical isolation between the AC power lines and an external remote control signal bus from which the optocoupler receives input signals; and (c) controllers (e.g., computers) coupled to the signal bus, each capable of asserting on the signal bus the control signals. The Liu Switch provides an xe2x80x9cON/OFFxe2x80x9d latching feature which allows random control by an unlimited number of external controllers and computers. In one implementation, the signal bus include an independent external common ground to which both the xe2x80x9cONxe2x80x9d signal and the xe2x80x9cOFFxe2x80x9d signal reference. In another implementation, separate independent external common ground references are provided for separate transmission and isolation between xe2x80x9cONxe2x80x9d-channel and xe2x80x9cOFFxe2x80x9d-channel on a four-wire external signal bus.
One aspect of the present invention provides a Liu Switch with no current leakage to the load in the xe2x80x9cOFFxe2x80x9d-state. Another aspect of the present invention provides a Liu Switch operating under a fully dynamic run mode during the xe2x80x9cONxe2x80x9d-state. The switch of the present invention operates from power received during the zero-crossing of each half-cycle of the input AC signal, thereby obviating a DC power supply. Thus, unlike any electronic switch in the prior art, the Liu Switch connects in series directly to the AC standard power line.
Another advantage of the present invention provides a static overcurrent tripping circuit and an automatic reset circuit in a Liu Switch. Thus, inexpensive independent overcurrent protection is provided at every load point. Independent overcurrent protection at every load point provides unsurpassed protection for property and lives.
Another advantage of the present invention is a universal Liu Switch capable of being directly connected to standard 120 volts, 220 volts or higher voltage AC power outlet.
According to another aspect of the present invention, an initialization and power recovery reset circuit (xe2x80x9cLiu""s Networkxe2x80x9d) is provided, including: a capacitor connected in series with a diode between a control terminal and ground, and a resistor connecting in parallel with the capacitor.
Another advantage of the present invention provides xe2x80x9coptional functionsxe2x80x9d in a Liu Switch. Such optional functions include providing a night light or a visible indicator on the Liu Switch. The night light or indicator function can be achieved using passive fluorescent materials, such as some chemical compounds of phosphates or sulfurs.
In one embodiment, an efficient LED is incorporated into a zero-crossing detecting circuit of the Liu Switch. In that embodiment, the forward voltage of the LED provides a threshold level to the zero-crossing circuit, and the LED provides a night light to illuminate the switch. In xe2x80x9cOFFxe2x80x9d state, the LED draws a current drawing less than 200 xcexcA.
Another advantage of the present invention provides a Liu Switch with a programmable dynamic threshold value for overcurrent tripping protection. The threshold value adapts to temperature, loading and environment conditions, and the configuration of the switch itself. LTS character especially can be use to create a smart terminals in electrical network.
The touch panels of the present invention is based on a discovery of an impedance effect related to the human body, called the xe2x80x9cLiu""s touch signal complementary effectxe2x80x9d (referred below as the xe2x80x9cLTSxe2x80x9d effect). The LTS effect result from the impedance properties of the human body, i.e., acting both as an impedance to ground and as an equivalent inductive signal source, over a wide range of environmental conditions. The LTS effect allows the Liu Switch to be reliably switched xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d over practically all environmental conditions by a human through the touch panels.
In that embodiment, a touch panel is electrically couple to the control circuit of the Liu Switch, such that when the touch panel is contacted by an external agent (e.g., a human operating the switch), an electrical path is created, and the LTS effect triggers the control circuit of Liu Switch.
According to on the aspect of the present invention, the xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d touch panels of the Liu Switch can be created to be different in position, color, shape, or texture, so as to ensure safety and precision operations. Such touch panels can be made from metal, resistive non-metals or conductors plated with an insulating material. In a Liu Switch of the present invention, different contact durations at the touch panels are required to trigger the Liu Switch""s xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d operations. In some embodiments, the touch panels are designed to prevent triggering by inadvertent contacts. For example, each touch panel can be provided a contact surface in a shallow depression.
In accordance with on the aspect of the present invention, the touch panels can be operated even when an operator is wearing gloves. This capability can be important in certain applications which require a piece of machinery to be disabled during emergency and within a very short time period. In such applications, the delay caused by an operator removing his or her work gloves in order to operate the switch is very undesirable. In accordance with another aspect of the present invention, when the first and second touch panels are touched substantially simultaneously, the Liu Switch resolves to an xe2x80x9cOFFxe2x80x9d state, thus preventing operating a load device inadvertently.
The Liu Switch of the present can control a resistive load, an inductive load, and some special loads, such as a fluorescent light, and can also be used to control mixed loads.